Unsubdividing

I want to minimize the number of vertices that are calculated for movement in animation while maintaining a higher level of detail, so I sculpted everything, retopologized, and used shrinkwrap and subdivision surface modifiers.

When I try to apply the shrinkwrap modifier, though, the details are lost and I get a warning: “Applied modifier was not first, result may not be as expected”

I’ve attempted what Fantasy Animations is suggesting in this video he’s made, but my model has something his doesn’t, because it didn’t unsubdivide as was demonstrated on the original, but did when I replicated it in a new file. I’ve also applied all the modifiers on the sculpted versions before applying everything else, but to no effect.

Is there something I’m missing? Where should I start looking for things that would interfere with the unsubdividing?

I think it was the mirror modifier. I removed it from both the sculpted and retopologized hands and it worked again.